The objective of this proposed research is to elucidate mechanisms by which specific genes control specific cytological events responsible for cell shape changes and morphogenesis. The system to be analyzed is the process of inversion in wild-type and mutant Volvox. At the end of cleavage, each Volvox embryo is a hollow sphere containing a syncitium of about 3000 somatic cells and 16 reproductive cells. But the sphere is inside-out with respect to adult conformation: flagella point to the interior and reproductive cells are on the surface. Inversion corrects this situation: two intersecting cracks appear at one end of the embryo, four flaps of cells bend outward and curl progressively over the surface of the embryo until the embryo has turned inside-out. Our preliminary studies suggest active shape changes in the cells at regions of maximum curvature as the motive force for the inversion process. Many single-locus mutant strains exist that arrest at specific stages of inversion. We propose: (1) To detail the mechanics of inversion by light and scanning electron microscopy of thick sections and scanning electron microscopy of whole embryos; (2) To describe with transmission electron microscopy the cytological bases for the cell shape changes at each stage of inversion, with emphasis on cytoskeletal elements and membrane-cytoskeleton interactions; (3) To examine the effects on inverting embryos of drugs that interfere with cytoskeletal organization and function; and (4) To analyze the nature of the cytological aberrations in mutant embryos that lead to inversion arrest. We hope thereby to deduce the role the wild-type alleles play in specific morphogenetic processes.